1. Field of the Invention
Embodiments of the present invention generally relate to optical sensing systems and, more particularly, to compensating for frequency fluctuations in source light used to interrogate optical sensors.
2. Description of the Related Art
Optical sensors can offer numerous advantages over conventional electrical and/or electromechanical sensing devices. For example, optical sensors typically have greater resistance to electromagnetic interference (EMI) than other types of sensors. Additionally, optical sensors tend to be small, lightweight, and physically robust. Consequently, optical sensors are commonly used in applications requiring resistance to a variety of harsh environmental conditions, such as downhole sensing applications.
Most optical sensors operate under the same basic principles. A sensitized region along an optical fiber is exposed to an environmental condition that modulates a light signal transmitted within the optical fiber. The modulation alters one or more parameters of the light transmitted within the optical fiber, such as amplitude, power distribution versus frequency/wavelength, phase, or polarization. Modulated light emerging from the fiber is analyzed to determine values indicative of the environmental condition. A wide variety of parameters may be measured using fiber-optic sensing techniques, such as strain, displacement, velocity, acceleration, flow, corrosion, chemical composition, temperature, and pressure, among others.
A fiber-optic interferometer sensor may be used to detect changes in light affected by an environmental condition as the light propagates along an optical fiber. A fiber-optic interferometer is typically formed by two reflectors, each placed at the end of a different optical path. One of the fiber-optic paths may be exposed to an environmental condition that alters a parameter of light transmitted through that path. Reflected light from each path may be recombined to mix coherently, thereby forming a “fringe” signal which is directly related to the difference in optical path lengths (i.e., the products of refractive index and physical length of the different paths). The fringe signals may be analyzed and correlated with the magnitude of the environmental condition. Fiber-optic interferometer sensors are typically used in applications where very sensitive measurements are required.
A Bragg grating sensor is an intrinsic optical sensor that operates by modulating the wavelength of a light field transmitted through an optical waveguide. A Bragg grating sensor comprises a tuned optical filter, or “Bragg grating”, imprinted upon the core of an optical waveguide coupled to a broadband light source. The Bragg grating is structured to reflect light within a narrow bandwidth centered at a Bragg wavelength corresponding to the spacing of the Bragg grating. If the Bragg grating sensor is strained, for example by stress or vibration, the Bragg grating spacing changes. This results in a shift in the reflected light wavelength, which can be measured and correlated with the magnitude of the stress or vibration. Bragg gratings may be paired within a length of optical fiber to form an FBG interferometer sensor. An FBG interferometer sensor generally provides greater sensitivity to strain changes in a length of optical fiber than a sensor utilizing a single Bragg grating.
Examples of an optical Bragg grating sensor are described in U.S. Pat. No. 6,422,084, entitled “Bragg Grating Pressure Sensor”, issued Jul. 23, 2002 to Fernald, et al.; and U.S. Pat. No. 6,452,667, entitled “Pressure Isolated Bragg Grating Temperature Sensor”, issued Sep. 17, 2002, to Fernald, et al., all of which are hereby incorporated by reference in their entireties.
Optical sensors have become increasingly popular in the petroleum industry due to their resistance to interference and tolerance for harsh environmental conditions. For example, optical sensors may be used as gravity meters in petroleum exploration to measure minute changes in the earth's gravitational field. Alternatively, optical sensors may be used as hydrophones in a water environment to measure shock waves reflected from hidden rock layers as part of a seismic survey process. Optical sensors are also used to monitor conditions within a well during or after drilling operations have been performed. For instance, optical sensors may be used in a well logging operation to take measurements of rock formations within a borehole. Alternatively, optical sensors may be used in drill-stem testing operations where pressure variations within a borehole are measured to determine the presence of oil reservoir rock in the surrounding strata.
As discussed above, an optical sensor may modulate the phase of light emitted by a light source responsive to an environmental condition. However, instabilities in the frequency of an interrogating light signal arriving at an optical sensor from a light source may cause variations in sensor signals. For example, fluctuations in the frequency of an interrogating light signal arriving at an interferometer or Bragg grating sensor may cause variations in the reflected light signal emitted by the sensor, resulting in undesirable noise.
Fluctuations in the frequency of an interrogating light signal arriving at an optical sensor are oftentimes due to light source output instabilities. However, in many environments, achieving a stable light source output is extremely difficult. For example, oil platforms and ships typically contain propulsion engines, diesel engine generators, hoisting systems, pumps, thrusters, and other such devices that generate significant vibrations during operation. A light source generally must be isolated from vibrations in order to obtain a light frequency output sufficiently stable to obtain accurate readings from typical Bragg grating and fiber-optic interferometer sensors. Consequently, in these and other vibration prone environments, damping units have been used to isolate light sources from low frequency vibrations, typically in the range of 1–100 Hz. However, low frequency damping units are expensive to build, very heavy, and oftentimes insufficient to completely eliminate vibration induced source frequency fluctuations.
Fluctuations in the frequency of an interrogating light signal arriving at an optical sensor may also be caused by changes to the light signal as it travels from a light source through an optical fiber to the sensor. For example, strain on an optical fiber due to environmental vibrations, pressure, and/or temperature changes may cause Doppler-shift effects, resulting in frequency fluctuations in a light signal passing through the optical fiber. Other environmental conditions may adversely affect the frequency of light emitted by a light source. For example, changes in the ambient bulk temperature and/or current applied to a light source may cause changes in a laser light source cavity, resulting in frequency fluctuations.
As discussed above, frequency fluctuations due to light source instabilities and/or changes to a light signal traveling through an optical fiber may result in sensor output variations and unwanted noise. These effects may significantly impair the accuracy and repeatability of an optical sensor system. In applications such as oil exploration and/or oil well monitoring, these effects may significantly increase the cost of drilling and oil extraction. In addition to frequency fluctuations, other parameters that may affect sensor accuracy are environmental parameters, such as temperature and acceleration, that affect the optical signal received from interrogated sensors.
One approach to compensate for such parameters is to utilize reference sensors (references) that are located at or near the measuring sensors (sensors) and, thus, subjected to the same environmental conditions. For example, U.S. Pat. No. 6,522,797, entitled Seismic Optical Acoustic Recursive Sensor System describes a time division multiplexed (TDM) interrogated marine seismic interferometric sensor array that employs reference interferometers that are co-located with the sensor interferometers and, therefore, should be subjected to the same temperature and acceleration. However, the reference interferometers may be made insensitive to parameters measured by the interferometer sensors (acoustic pressure in this case). The referenced patent describes a method for removing sensor sensitivity to temperature and acceleration by subtracting fringe signals received from the reference interferometers from corresponding signals received from the sensor interferometers. One problem with this approach, however, is that the sensor interferometers are still sensitive to both source frequency fluctuations and lead fiber length fluctuations.
Accordingly, there is a need to eliminate or reduce the effects of frequency fluctuations in an interrogating light signal arriving at an optical sensor in order to improve sensor accuracy and repeatability.